It is well known that electro-optic modulators are fabricated using a substrate of electro-optic material by forming an optical waveguide in the substrate and depositing thin film electrodes on the surface of the substrate. When a potential difference is established between the electrodes, an electric field is created within the substrate and this influences the refractive index of the electro-optic material. Accordingly, when light is propagated through the optical waveguide, its phase varies in dependence upon the magnitude of the imposed electric field.
Electro-optic modulators are typically designed for very high speed modulation of signals in optical fiber communication systems. As such, they are designed to be used in a 50 Ohm environment and have a 50 Ohm input impedance. To be used in a 50 Ohm system, the electrodes are designed as a 50 Ohm transmission line fabricated on the surface of an electro-optic crystal containing optical waveguides. This transmission line is typically terminated in its characteristic impedance (50 Ohms) so that the electrical signal does not reflect from the end of the transmission line.
The electric field generated between the electrodes modulates the index of refraction of the crystal in the optical waveguide which causes the phase of the light in the waveguide to change in proportion to the applied electric field. The electrical transmission line and the optical waveguide in this type of modulator can be designed so that the velocity of the electrical wave propagating in the electrical transmission line matches the velocity of propagation of the optical wave travelling in the optical waveguide. This velocity matching enables this type of structure to have very high sensitivity and very high bandwidth.
What is needed is an electro-optic modulator with a high input impedance and high common mode rejection ratio (CMRR). CMRR is not generally a consideration in conventional high-speed communication modulators.